An OTDR is an apparatus for injecting an optical pulse into an optical fiber, measuring intensities of reflected optical signals while the injected optical pulse passes through the optical fiber, and identifying characteristics of the optical fiber. That is, the OTDR is an apparatus for injecting an optical pulse into an optical fiber, analyzing a distance distribution of the intensity of radiation returned after being reflected from each point in a longitudinal direction, and measuring loss of the optical fiber, a distance to a connection point, connection loss, the amount of reflection from the connection point, a distance to a breakage point when the optical fiber has been broken, etc.
The reflection in the optical fiber is mainly caused by Rayleigh Backscattering or Fresnel reflections. The Rayleigh Backscattering occurs due to density in the optical fiber and refractive displacement caused by structural change. The Fresnel reflections occur due to connection or link between the optical fibers or difference of refractive index in an end of the optical fiber.
FIG. 1 is a block diagram showing the basic structure of an OTDR.
A Laser Diode (LD) 120 generates an optical pulse according to an electrical pulse generated by a pulse generator 110. The generated optical pulse is injected into an optical fiber 170 to be measured through an optical coupler 130.
Optical signals reflected from the optical fiber 170 are inputted to a Photo Diode (PD) 140 via the optical coupler 130. The Photo Diode 140 outputs electrical current proportional to intensities of the inputted optical signals.
The electrical current is converted into voltage and amplified by a Trans-Impedance Amplifier (TIA) 150, which is inputted to a microprocessor 100 via an Analog-to-Digital Converter (ADC) 160.
The microprocessor 100 processes a measured waveform of the voltage, displays the measured waveform as a function according to a distance from an injection point of the optical pulse on a display unit 180, and understands loss of the optical fiber, a distance to a place at which the loss of the optical fiber has occurred, etc., from the waveform. The measured waveform is called “OTDR trace”.
For exact measurement in this OTDR, a method for improving a Signal-to-Noise Ratio (Hereinafter, referred to as SNR) may use a method for increasing a pulse width of an optical pulse. As the pulse width of the optical pulse increases, energy of inputted light increases. Therefore, intensities of reflected optical signals increases, so that the SNR is improved. However, it is impossible to detect abnormality of an optical fiber occurring an interval smaller than the pulse width. Consequently, a resolution may deteriorate.
Accordingly, a method (Hereinafter, referred to as average measurement method) for obtaining an average through several measurements has been used as a method capable of improving the SNR without deteriorating the resolution.
When the reflected original signal is referred to as s(t), an ith measured value is referred to as ri(t), and noise is referred to as ni(t) (i=1, 2, . . . , N), a relation between the three factors may be expressed by equation 1 below.ri(t)=s(t)+ei(t) (i=1, 2, . . . , N)  Equation 1
In equation 1, when it is assumed that ei(t) is independent for i, an average is 0, and a dispersion is σ2, an average of r(t) obtained by averaging N number of ri(t) may be expressed by equation 2 below and a dispersion (intensity of noise) of r(t) may be expressed by equation 3 below.
                                                                        E                ⁢                                  {                                      r                    ⁡                                          (                      t                      )                                                        }                                            =                            ⁢                              E                ⁢                                  {                                                            1                      N                                        ⁢                                                                  ∑                                                  i                          =                          1                                                N                                            ⁢                                                                                          ⁢                                                                        r                          i                                                ⁡                                                  (                          t                          )                                                                                                      }                                                                                                        =                            ⁢                              E                ⁢                                  {                                                            s                      ⁡                                              (                        t                        )                                                              +                                                                  1                        N                                            ⁢                                                                        ∑                                                      i                            =                            1                                                    N                                                ⁢                                                                                                  ⁢                                                                              e                            i                                                    ⁡                                                      (                            t                            )                                                                                                                                }                                                                                                        =                            ⁢                              s                ⁡                                  (                  t                  )                                                                                        Eqution        ⁢                                  ⁢        2            
                              E          ⁢                      {                                          [                                                      r                    ⁡                                          (                      t                      )                                                        -                                      s                    ⁡                                          (                      t                      )                                                                      ]                            2                        }                          =                              E            ⁢                          {                                                [                                                            1                      N                                        ⁢                                                                  ∑                                                  i                          =                          1                                                N                                            ⁢                                                                                          ⁢                                                                        e                          i                                                ⁡                                                  (                          t                          )                                                                                                      ]                                2                            }                                =                                    σ              2                        N                                              Equation        ⁢                                  ⁢        3            
As expressed by equation 3, it can be understood that the average measurement method of obtaining an OTDR trace through an average with N measurements has noise reduced by 1/N times at the cost of N times longer measurement time, as compared with a method of obtaining a trace through only one time measurement.
Recently, in order to improve the SNR, various methods have been proposed, which modulates optical pulses with a specific code, injects the modulated optical pulses into an optical fiber, and restores reflected signals by means of signal processing techniques, in addition to the average measurement method. From among the proposed methods, a method using a complementary code of Golay has been experimentally proved to improve the SNR as compared with the average measurement method as described above.
The method using the complementary code of Golay improves the SNR when a code length exceeds a predetermined length, but a resolution degrades twice due to a decoding characteristic of the Golay code, as compared with the average measurement method. Therefore, practical SNR performance is reduced by 3 dB than a proposed theoretical value.